The invention relates to a composition for generating smoke, the composition comprising one or more chlorine compounds having a chlorine content of at least 56% by weight and a mixture of one or more metal oxides and one or more metals.
Such a composition is disclosed by DE-A-2,451,701, said composition consisting of a chlorine-containing compound which contains 50-70% by weight of chlorine, a metal and a metal oxide. The metal can be zinc, aluminium, magnesium, titanium, iron, and aluminium-magnesium alloy or an alloy-like compound such as calcium silicide or iron(II) silicon. According to p. 11, second paragraph, the metal oxide can be zinc oxide, magnesium oxide or iron(III) oxide, the compositions according to the examples always containing zinc oxide. The function of the metal oxide is not reported. According to DE-A-2,451,701, the chlorine-containing compound is preferably a chlorine-containing polymer. Such a polymer has the advantage, compared with the customary hexachloroethane, that the polymer hydrolyses only slowly to form HCl and other decomposition products. According to the description, HCl and said decomposition products react exothermally with the metal present in the composition, as a result of which autoignition of the composition may take place or as a result of which the composition may even explode. Thus an embodiment of the composition according to DE-A-2,451,701 (p. 8) consists of 35% by weight of a chlorine-containing paraffin wax, 20% by weight of aluminium and 45% by weight of zinc oxide, i.e. a composition in which the molar fraction of aluminium, based on the total number of moles of aluminium and zinc oxide, is 0.57. This composition, however, is very difficult to ignite, whereas a comparable composition comprising hexachloroethane instead of the chlorine-containing paraffin wax will explode under the same conditions. It is also reported that compositions comprising hexachloroethane, zinc oxide, and an amount of 15% by weight of a metal explode readily, so that in compositions of this type the amount of metal to be used is greatly restricted.
Compositions for generating smoke are generally used to mask objects and people, the smoke reducing the contrast between the object or the person and the background to such an extent that the object or the person is scarcely visible, or is not visible at all.
The most frequently used composition for generating smoke consists mainly of hexachloroethane and zinc oxide. The smoke is generated by combustion of the composition, forming hygroscopic particles of zinc chloride. These particles absorb moisture from the atmosphere, thus producing smoke. For example, FR-A-2,249,590 describes compositions which contain hexachloroethane, zinc oxide, an organic binder and calcium silicide, magnesium silicide or aluminium silicide. U.S. Pat. No. 3,625,855 describes compositions which comprise a chlorine-containing compound, for example hexachlorobenzene, zinc oxide, magnesium and a binder. An important drawback of such compositions is that zinc chloride is toxic and, in particular, may cause pulmonary oedema, due to the fact that zinc chloride reacts with water in the lungs to form hydrogen chloride which attacks the lungs. The formation of toxic metal chlorides can be avoided, for example, by preparing a composition which generates potassium chloride or magnesium chloride instead of zinc chloride. In this case, combustion gives rise to hygroscopic particles of magnesium chloride or potassium chloride. Magnesium-containing compositions have the drawback, however, that they burn too rapidly and at an excessively high temperature, and these compositions can therefore not be used in a container such as a smoke hand grenade or smoke canister, since a combustion time of xc2x11 minute is desirable for such containers. A composition which generates potassium chloride in particular has the drawback that the smoke formed is of considerably lower quality (high transmission) than if a composition comprising hexachloroethane and zinc oxide is burnt.
GB-A-2,056,632 describes a smoke grenade comprising a composition which contains hexachloroethane, titanium dioxide and aluminium and wherein the molar fraction of aluminium, based on the total number of moles of aluminium and titanium dioxide, is between 0.05 and 0.75. Titanium dioxide is used, in particular, because it is much cheaper than titanium (p. 3, lines 50-52 and p. 5, lines 88-90). In addition it is claimed that the use of titanium dioxide has the advantage, compared with zinc oxide, that the smoke formed by combustion of the composition which contains titanium dioxide has a lower condensation point. Consequently, the composition according to GB-A-2,056,632 can be used, for example, in snow. A further advantage of the composition which contains titanium dioxide is that this composition is effective not only in the visible range of the electromagnetic spectrum, but also in the infrared range.
In addition, the German Patent Application 2,250,102 describes a candle for dispersing fog, which comprises 30-50% by weight of hexachloroethane, 45-65% by weight of magnesium and 3-7% by weight of magnesium oxide, the sum of the constituents being 100% by weight. The molar fraction of magnesium, based on the total number of moles of magnesium and magnesium oxide, is therefore approximately from 0.11 to 0.15. The various constituents are present in the candle in finely dispersed form, for example as a powder. The candle functions as follows. When the candle is ignited, the magnesium and the magnesium oxide react with hexachloroethane, primarily forming magnesium chloride particles. These particles are highly hygroscopic, and will therefore absorb the moisture present in the atmosphere, with the result that the fog disappears.
It is known from xe2x80x9cPropellants, Explos., Pyrotech.xe2x80x9d, vol. 9(3), pp. 108-114 (1984) that certain metal oxides, such as zinc oxide, magnesium oxide, copper oxide and aluminium oxide make it more difficult to ignite a composition of hexachloroethane and xe2x80x9csiluminxe2x80x9d (silumin is a group of alloys of aluminium and silicon which contain approximately 12% of silicon and have a density of from approximately 2.63 to 2.65) or retard the combustion of such a composition. For example, a composition consisting of approximately 26% by weight of magnesium oxide, approximately 51% by weight of hexachloroethane and approximately 22% by weight of silumin was virtually impossible to ignite. When the amount of magnesium oxide was reduced to approximately 12% by weight, combustion did take place, but the burning rate of this composition was much lower than that of a composition which did not contain any metal oxide. An earlier study by the same authors [xe2x80x9cPropellants, Explos., Pyrotech.xe2x80x9d, vol. 9(2), pp. 64-71 (1984)] showed that compositions comprising hexachloroethane, silumin and more than 4% by weight of magnesium oxide are difficult to burn. The combustion mechanism of such compositions is claimed to comprise a reaction in which silumin and hexachloroethane react with one another to form an xe2x80x9cignition catalystxe2x80x9d. This catalyst is alleged to be deactivated by the metal oxide, for example magnesium oxide, with the formation of the metal, for example magnesium, from the metal oxide, giving rise to retardation of the combustion of the composition. It follows from the mechanism put forward by the authors that the metal formed by deactivation of the catalyst cannot play an active role during the combustion of the composition. Moreover, this study has shown that compositions for generating smoke may only contain small quantities of a metal oxide, such as magnesium oxide or zinc oxide, since with larger quantities, for example a quantity of more than 4% by weight, the compositions are very difficult or impossible to ignite and burn.
A composition has now been found for generating smoke which is not potentially explosive, with which no or virtually no toxic reaction products result from the combustion of the composition, and with which there is a high level of smoke production (high efficacy) with respect to the quantity of the composition employed. The burning rate of the composition according to the invention is such that the composition can be used in a container such as, for example, a smoke canister or a smoke hand grenade. At the same time, the burning temperature of the composition according to the invention is lower than that of the conventional compositions, with the result that igneous phenomena, which might lead to a fire in the area surrounding the location where the composition is ignited, will occur less rapidly. The smoke produced upon combustion of the composition according to the invention also has the advantage that this smoke is effective in the infrared range, since the combustion gives rise to magnesium chloride particles which react exothermally with moisture. The invention therefore relates to a composition for generating smoke comprising one or more chlorine compounds having a chlorine content of at least 56% by weight and a mixture of one or more metal oxides and one or more metals, wherein the metal oxide is an alkaline-earth metal oxide and the metal is an alkaline-earth metal, and wherein the molar fraction of alkaline-earth metal, based on the total number of moles of alkaline-earth metal and alkaline-earth metal oxide is between 0.33 and 0.67.